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Problem 22

Number theory Difficulty 1.1 Multiple choice CEMC Cayley · Canada · 2021

The ones (units) digit of 542 is 2. When 542 is multiplied by 3, the ones (units) digit of the result is

Pick one

Official solution

Since $542 \times 3 = 1626$ , the ones digit of the result is 6.

Note that the ones digit of a product only depends on the ones digits of the numbers being multiplied, so we could in fact multiply $2 \times 3$ and look at the ones digit of this product.

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